The present invention relates to a pulse generator for electroerosive processing by means of an electrical discharge between a tool electrode and a workpiece electrode, the generator consisting of a main voltage source, an adjustable control oscillator, and a plurality of impulse amplifier switching circuit stages.
As in finishing technology in general, so also in spark erosion processing is there the constant desire for increases in rate of erosion for the process without having to accept therewith excessive increase in tool wear. It is known that the impulses which provide the highest rate of erosion are those which during the short discharge time are able to provide the greatest degree of energy conversion; that is, pulses which within the short discharge period can create an especially strong discharge current. In electroerosive processing, especially in electroerosive cutting, there is a special need for high current short pulses, since in general one need be less concerned about wear of the electrode. The wear can be much more readily compensated for by an increase in the wire travel speed. In the application of high current short impulses, further advantageous use can be made of a shift in the physics of the removal mechanism from a melt removal more to one of removal by vaporization. In connection with the last mentioned type of removal there is the advantage that relatively little degradation of the workpiece surface results, since there arises a very sharp temperature gradient between the material involved in the vaporization and the remainder of the workpiece material. In the use for spark erosion cutting, there appears in practice today the further difficulty that the current impulses, which last for a number of microseconds, have parameters influenced during the course of the discharge by the unavoidable movements (swings) of the sensitive wire electrode. By instituting high current short impulses in the range of one microsecond, elimination of this disturbing influence of the wire on the course of the discharge can be achieved, since the discharge occurs in much too short a time as compared to the movement conditions for the electrode.
It is known in the field of spark erosion to realize the electrical switching circuits of the pulse generator by means of transistors or thyristors. The development of such pulse generators especially designed for spark erosion sinking began in about 1960 and was then already characterized in U.S. Pat. No. 3,292,040, (Ullmann et al). These have in the meantime become public property and are appropriately described even in the industry standards as "static impulse generators". Up to the present point in time, the production of high performance short impulses by means of semiconductors has always remained an unsolved problem, and has commonly been avoided by supplementation with storage means. That is, production transistors have the undesirable characteristic of exhibiting during the course of the impulse an uncontrollable current cut-off delay period. There are indeed known some anti-saturation switch circuits, which bring improvements in the field of signal processing, but which have an absolutely insufficient performance for power circuits. Such switches would also be unusually expensive. Thyristors, on the other hand, require very special measures just to bring them out of their conductive state at all. Besides, these elements are today subjected in manufacturing use to constraints on the operating frequency, the current rise-time, and the voltage rise-time which are too demanding. One can find such arrangements disclosed, for example, in Radartechnique (Radar Handbook, page 7-85, Merril Skolnik, 1970 Publ. McGraw-Hill). There also came to be known other switching arrangements which for high current short impulses made use of so-called hydrogen-thyratrons such as, for example, in an erosion generator A671.45 described in the periodical "Electrochemistry in Industrial Processing and Biology" No. 6, Nov./Dec. 1975, page 83. These elements do indeed offer very good impulse characteristics, but are subject to the known shortcomings of electron tube technology: excessive operating voltage of several thousand volts, long warmup time, and short lifetime. Besides, thyratrons are too limited in regard to frequency and in principle require for cut-off the same measures as do thyristors.
The invention relates, then, to a generator for electroerosive processing by means of an electrical discharge between a tool electrode and a workpiece electrode, the generator consisting of an oscillator circuit which cyclically switches a current source to the discharge path through power transistors, characterized in that for increasing the removal rate during the erosion process, the power transistors are so controlled through impulse transformers that only so much control current can reach the control electrode of the power transistors as is necessary for suitable forward bias for these elements, in that the surplus control current is transformed up to the suitable output voltage and shunted through diodes to the power electrodes of the power transistors. The invention is further characterized in that the primary winding of the impulse transformer is connected to the first impulse amplifier switching stage of the oscillator switching circuit, that all further impulse amplifier stages are controlled in series with the power transistor of the first one, and that these are connected through current limiting resistors to the output terminal of the generator. The invention is further characterized by the use of compensation circuits (RC elements or LR elements) for compensating for the capacitances and inductances inherent in the transfer between pulse generation and the discharge path, taking into account the arrangement of the cable itself.